Field of the Disclosure
The present disclosure relates to a display device, and more particularly, to an electroluminescent display device and a driving method of the same.
Description of the Background
Electroluminescent display devices can be divided into inorganic light emitting display devices and organic light emitting display device by which material is used for an emission layer. Among them, an active matrix-type organic light emitting display device includes an organic light emitting diode (OLED) which emits light by itself and which is a typical example of the electroluminescent light emitting diode. In addition, the active matrix-type organic light emitting display device has advantages of quick response, high luminous efficiency and brightness, and a wide viewing angle.
The OLED, which is a self emitting element, includes an anode electrode, a cathode electrode and an organic compound layer positioned therebetween. The organic light emitting display device include pixels which are arranged in a matrix form, each pixel having an OLED and a driving Thin Film Transistor (TFT), and the organic light emitting display device adjust brightness of an image displayed by the pixels according to a gray level of image data. According to a voltage applied to a gate electrode and a source electrode of the driving TFT (the voltage which is referred to as a “gate-source voltage”), the driving TFT controls a driving current flowing in an OLED. According to the driving current, luminous power and brightness of the OLED is determined.
When a driving TFT operates in a saturation region, a driving current flowing between a drain and a source of the driving TFT is generally represented as below:Ids=½*(u*C*W/L)*(Vgs−Vth)2 
Wherein u denotes electron mobility, C denotes a capacitance of a gate insulation layer, W denotes a channel width of the driving TFT, L denotes a channel length of the driving TFT, Vgs denotes a gate-source voltage of the driving TFT, and Vth denotes a threshold voltage of the driving TFT. Depending on a pixel structure, the gate source voltage Vgs of the driving TFT may be a differential voltage between a data voltage and a reference voltage. As the data voltage is an analog voltage corresponding to a gray level of image data and the reference voltage is a fixed voltage, the gate-source voltage Vgs of the driving TFT is programmed (or set) according to the data voltage. The driving current Ids is determined according to the programmed gate-source voltage Vgs.
Electrical characteristics of a driving TFT, such as the threshold voltage Vth and the electron mobility u, are factors that determine a driving current Ids, and thus, driving TFTs in all pixels should have the same electrical characteristics. However, the electrical characteristics may be different among pixels for various reasons, such as process variation and driving time increase. Such a deviation in electrical characteristic of a driving TFT may result in degrading image quality and reduce the lifespan of a device.
To compensate for a deviation in electrical characteristic, external compensation techniques are used. The external compensation techniques is implemented to sense a driving current Ids dependent upon a driving TFT and modulate data of an input image based on a sensing result so as to compensate for a deviation in electrical characteristics between pixels.
When electrical characteristics of a driving TFT in a specific pixel is being sensed, a driving Ids is not flowing into an OLED but applied to an external sensing circuit to thereby enable an OLED to emit light. This is to increase accuracy of sensing. As the electrical characteristics of a driving TFT are sensed with an OLED in a non-light emitting state, the sensing operation is performed in a specific time when an image is not displayed. In other words, the sensing operation is performed in a booting time which lasts until a screen turns on after system power is applied, or may be in a power-off time which lasts until the system power is off after the screen is turned off.
An existing electroluminescent display device splits an operation of sensing of a threshold voltage of a driving TFT and an operation of sensing of electron mobility of the driving TFT. After a threshold voltage of a driving TFT in every pixel of the existing electroluminescent display deice is sensed, electron mobility of a driving TFT in every pixel is sensed. If threshold voltage and electron mobility are sensed separately, it takes long time to perform a sensing operation and prolong a booting time and a power-off time, resulting in a degradation of performance of the display device.